139736eo.pdf (20MB) - Japan Oceanographic Data Center

scale wind phenomenon that occurs with sufficient frequency and influence on the local weather(PERRONE, 1981). “Shamal” is used in the meteorological context to refer to seasonal northwesterlywinds that occur during winter as well as summer.The winter Shamal occurs mainly from November to March and is associated withmid-latitude disturbances travelling from west to east. It usually occurs following the passage of acold front and is characterized by strong northwesterly winds (mainly during December to February).PERONNE (198 1) mentions that the winds at most locations in the Gulf exceed 20 knots less than 5%of the winter. Although infrequent, the winter Shamal is not insignificant from an operational point ofview, because it sets in with great abruptness and force; the March 1983 oil spill in the Arabian Gulfoccurred during a winter Shamal. The summer Shamal occurs with practically no interruption fromearly June through July and is associated with the relative strengths of the Indian and Arabian thermallows. When viewed from the point of wind strength and the associated weather conditions, however,the summer Shamal is not as important as the winter Shamal.Although the winter Shamal is a relatively rare event, typically occurring only once or twiceeach winter, it brings some of the strongest winds and highest seas of the season to the Gulf region.Usually the Shamal occurs first in the northwest part of the Gulf and then spreads south and eastbehind the advancing cold front. It takes up to 12 to 24 hours for the Shamal to spread from thenorthwest comer of the Gulf to the soutnern part. The onset of the Shamal is difficult to predict,mainly because of the difficulty in forecasting the associated upper air pattern. Once the Shamal hasbegun, it may subside within 12 to 36 hours after the passage of the cold front or it may persist forthree to five days. The relationship between the surface and upper air patterns determines whichsequence is most likely to occur. Once the Shamal begins, the wind direction is strongly influenced bythe coastal orography. In the northern part of the inner Gulf, the Shamal winds generally blow fromnorth to west-northwest. In the middle parts of the Gulf, Shamal winds tend to be fromwest-northwest to northwest. On the southeast coast of the Gulf, the winds are westerly. In the Straitof Hormuz area, the Shamal winds are generally from the southwest. The speed of Shamal windsgenerally ranges from 20 to 40 knots.The Shamal winds persist for three to five days with gale force and blow from northwest overthe whole Gulf. The longer duration Shamals occur when the upper air trough stalls over the Strait ofHormuz. Because of the large pressure gradient between the low (over the Gulf of Oman) and thehigh (over Saudi Arabia), the Shamal winds are strongest in the southern and southeastern parts of theGulf. Average wind speeds in the southern and southeastern parts of the Gulf range from 30 to 40knots, with peak winds in excess of 50 knots not uncommon. Winds over the northern part of theGulf, on average tend to be 5 to 15 knots less than the above values.According to PERRONE (1981), two areas of the Gulf experience stronger than averageShamal conditions (Fig. 2). One area is near the Qatar Peninsula, and another is near Lavan Island.STORM SURGESBased on the linearized versions of the shallow water equations, MURTY and EL-SABH(1984) numerically simulated the storm surges in the Gulf using finite difference scheme. Figure 3shows schematically the spatial structure of the storm as it advances one grid space in the x-direction.This storm has a life of about one day and is supposed to represent a short duration winter Shamal.Figure 4a shows for selected locations the computed profiles of tide (with three semi-diurnal and threediurnal constituents included), tide plus storm surge and storm surge alone.MURTY and EL-SABH (1984) also made additional simulations with the same winddirections, but for a longer duration (4 to 5 days) Shamal. Figure 4b shows the results of thesimulations when the wind constantly blows from the positive y-direction, while Figure 5 shows plotsat four different times of the horizontal distribution of the storm surge heights. From these varioustypes of plots, it can be seen that significant positive and negative storm surges can occur in theArabian Gulf. The tide is also important.238